The present invention relates to a series voltage regulator as in the introductory part of claim 1.
A conventional series voltage regulator, as known from FIG. 1 of DE-OS No. 2,700,111 and as shown in FIG. 1 of the subject application, is used for supplying a load with a stabilized direct voltage. In order for the nominal output voltage of the series voltage regulator to be obtained, its input voltage must exceed a certain critical level. If the input voltage falls below this critical level, the differential amplifier drives the regulating transistor into a saturated state. Due to the low collector-to-emitter saturation resistance of regulating transistor T, interference voltage, for example interference alternating voltage, may reach the regulator output virtually unimpeded in this saturation state. Suppression of interference thus occurs only in the normal voltage range, i.e. at input voltages higher than the critical level at which the nominal voltage can be reached on the output side.
In various applications, e.g. for car radios, suppression of the alternating voltage portions of the input signal is necessary in addition to the stabilization of the direct voltage mean value at the output of the series voltage regulator.
For this purpose, series voltage regulators, as known from FIGS. 2 and 3 of DE-OS No. 2,700,111 and from FUNK-TECHNIK 1965, No. 23, pages 947 to 950, comprise an RC low-pass filter, with the R thereof being constituted by the resistance of the collector-to-emitter path of the regulating transistor and the C thereof being constituted by a capacitor connected in parallel to the output of the series voltage regulator. When the capacitance value of this capacitor is fixed, the filtering effect of such a low-pass filter decreases as R decreases. Due to the fact that in case of these known series voltage regulators the low-pass filter is dimensioned for the normal voltage range, in which the resistance R of the collector-to-emitter path of the regulating transistor is high, the filtering effect of this low-pass filter deteriorates significantly when the regulating transistor goes into the saturation state in the undervoltage range on the input side of the series voltage regulator and the resistance R constituted by said regulating transistor still is only very low. Thus, these known series voltage regulators fulfill the suppression of interference at the most in the range of normal input voltages, but not in the undervoltage range on the input side, in which the output voltage no longer reaches its nominal value and the regulating transistor goes into the saturation state.
Suppression of interference also in the undervoltage range may be achieved by arranging a conventional RC low-pass filter subsequent to such a series voltage regulator. However, this involves additional expenditure and additional power dissipation in the resistor of this additional RC low-pass filter. Discrete circuits of a transistor/Zener diode/capacitor combination lead to unsatisfactory approximate solutions.
A series voltage regulator as it is known from U.S. Pat. No. 3,916,294, comprises a first capacitor on the input side, which serves for attenuating interfering alternating voltage portions of the input voltage. A second capacitor is connected in parallel to a Zener diode which is connected on the one hand to the emitter of a transistor constituting a differential amplifier and on the other hand to a voltage divider which is connected in parallel to the output of the series voltage regulator. This second capacitor serves for attentuating alternating voltage portions present in the output voltage of the series voltage regulator in order to reduce their effect on the regulation of the regulating transistor. If in case of this known series voltage regulator the regulating transistor comes into the saturation state in undervoltage operation on the input side, the interference signals reaching this regulating transistor reach the output of the series voltage regulator virtually unimpeded.